Up to a few years ago almost every light duty truck, automobile and most boat and utility trailers were equipped with a combination tail, turn and brake light fixture on both rear corners that used an 1157 bulb. The bulb fit a special socket that activated this double-filament bulb. It was a reliable system, easy to understand and often just as easy to repair when the lights stopped working correctly.
Most of the time, when you notice your rear tail/turn light malfunctioning, the problem is the bulb rather than the socket so always check the bulb first. Remember, the 1157 bulb has dual filaments, one for the tail light function, the other for the brake/turn light function. If either of these are broken, it could easily affect the proper function of the other, unbroken filament. Change the bulb.
Most tail/turn light assemblies are not perfectly water or air tight. Moisture can get inside, condense on the metal fixture and cause corrosion. Remove the lens of the assembly and the light bulb, then use a small piece of Emory cloth, rolled up and inserted into the socket to shine up the interior and remove corrosion. Use a small screwdriver to scratch the two positive contacts in the bottom of the socket.
An 1157 socket will have two wires connected to its back side. Both of these are positive wires. Use a 12V, probe-type test light to ensure the positive wires are hot when the light is activated. If you find a problem, closely exam the wire and connection until you locate the exact problem.
An 1157 socket grounds through the metal frame of the light assembly either with a wire or through the mounting screws that hold the assembly to the vehicle or trailer. If the lights on your vehicle or trailer are acting "weird," such as both turn signals blinking at once, the tail lights turning off when the brakes are applies or going bright then dim, suspect a faulty ground. Inspect the ground wire or check the grounding screws for corrosion.
